Water dispersible polyisocyanates are commonly applied in the adhesives based on the aqueous resins as crosslinking agents. Particularly, the isocyanates are highly reactive to the compounds having active hydrogen groups so as to form, for example, urethane, urea, amide, allophanate, and biuret products. Thus, the polyisocyanates are advantageous to the molecular growth and crosslinking of the resins so as to extensively improve their adhesion, heat resistance, and mechanical performance when being used as adhesives. The characteristic of high reactivity of the polyisocyanates allows them to be an excellent crosslinking agent for the aqueous resins.
The preparations and applications of the water dispersible polyisocyanates are known and have been disclosed. For example, U.S. Pat. No. 4,663,377 discloses the utilization of one or more aliphatic polyisocyanates having an average NCO functionality of about 2.0 to 3.5, to react with hydrophilic materials of polyoxyalkylene monols, such as poly ethoxylated ether monol exemplified in the examples provided thereof. The obtained water dispersible polyisocyanate preparation has an NCO % of 18.4% and a viscosity of 2500 cps/23° C. The water dispersible polyisocyanate preparation in an amount of 2 wt % to 10 wt % is added to an aqueous adhesive so as to enhance the adhesive performance.
U.S. Pat. No. 5,200,489 relates to the use of a polyoxyalkylene monol to modify polyisocyanates based on 1,6-hexamethylene diisocyanate (HDI) or a mixture of HDI with up to 70% by weight, based on the weight of the diisocyanate, of another diisocyanate, to obtain a water dispersible polyisocyanate composition having an average NCO functionality of 2 to 6, and further provides a method for testing the water dispersibility of the polyisocyanate composition. The test results show that the polyoxyalkylene monol in an amount of about 10 wt % and having a molecular weight of 200 to 1000 g/mole is more favorable to improve the water dispersibility of the polyisocyanate composition.
U.S. Pat. No. 5,252,696 teaches a water dispersible polyisocyanate mixture obtained by reacting an aliphatic polyisocyanate having an NCO functionality of about 2.1 to 4.4, such as HDI trimer, with a polyoxyalkylene monol containing 5 to 9.9 ethylene oxide (EO) units, at an NCO/OH equivalent ratio of 4:1 to 120:1. One synthesis example relates to the addition of poly ethoxylated ether monol to HDI trimer for conducing reaction so as to obtain a product having an NCO % of 17.3% and a viscosity of 3050 cps/23° C. The obtained water dispersible polyisocyanate mixture is then added to an aqueous acrylic dispersant bearing a hydroxyl group, so as to provide good surface gloss and solvent resistance.
U.S. Pat. No. 6,221,995 discloses a modified polyisocyanate which has a high emulsifying/dispersing ability in an aqueous resin composition and can be advantageously used as a curing or crosslinking agent. The modified polyisocyanate is prepared by reacting an HDI-based isocyanurate group-containing polyisocyanate and a nonionic emulsifying agent, poly ethoxylated ether monol. U.S. Pat. No. 6,221,995 mentions that the hydrophilic structure of a polyoxyalkylene monol is expressed by a hydrophile-lipophile balance (HLB), and indicates that a water dispersible polyisocyanate synthesized from a polyoxyalkylene monol having a HLB of 17 or less (about 5 to 17) provides better performance when used in aqueous coatings, inks, and adhesives.
In view of the known technologies, the water dispersible polyisocyanates are normally prepared by reacting an aliphatic polyisocyanate based on HDI trimer with a nonionic polyoxyalkylene monol having a low molecular weight. Nonetheless, the reaction rate of a polyoxyalkylene alcohol with a polyisocyanate is slow and normally need to be accelerated by adding catalysts. Moreover, the water dispersible polyisocyanates of the prior art cannot immediately enhance the initial adhesion between rubbers and rubbers when being used in the adhesion of rubber substrates.
The inventors of the present invention have found that the reaction rate of polyoxyalkylene amines or mixtures of polyoxyalkylene amines and polyoxyalkylene alcohols with polyisocyanates is faster than that of polyoxyalkylene alcohols with polyisocyanates, which is advantageous to prepare water dispersible polyisocyanate compositions, especially water dispersible polyisocyanate compositions bearing urea or biuret. In addition, the water dispersible polyisocyanate composition of the present invention is more favorable for use in an aqueous resin as a crosslinking agent, compared with those prepared by the prior art technologies. Particularly, the aqueous resin adhesives crosslinked with the water dispersible polyisocyanate composition of the present invention have increased mechanical properties, such as adhesive ability, including the initial and aging adhesion, and heat resistance.
Although in view of the conventional technologies, the water dispersible polyisocyanates (WDPIs) are normally prepared by reacting an aliphatic polyisocyanate based on HDI trimer with a nonionic polyoxyalkylene monol and/or polyol, U.S. Pat. No. 5,202,377 discloses a WDPI made from an aliphatic polyisocyanate and a polyoxyalkylene monoamine. However, this kind of WDPI needs high boiling-point solvent (N-methyl pyrrolidone) to suppress the viscosity built up due to the formed urea linkages. This problem had kept people who are skilled in the field not using polyoxyalkylene amine to prepare WDPIs. Moreover, no reference had mentioned how to solve the problem without adding solvents in the process of making this kind of WDPIs until the inventors of the present invention utilized a known chemistry—biuretization of urea.
In general, the viscosity of a composition would increase with molecular weight (MW). Biuretization converts urea groups into biuret groups with isocyanates, and biuret will further turn into higher urets (herein called “biuret derivatives”, such as tri-uret and tetra-uret) if the isocyanates have enough reactivity toward biuret and its derivatives. This reaction increases the MW of the polyisocyanate composition and results in an expected viscosity buildup. In the present invention, the inventors provide a new kind of WDPIs also made from aliphatic polyisocyanates and polyoxyalkylene amines but without the foreseeable problem. No solvent is required to reduce the viscosity in the preparation of this new kind of WDPIs. The fluidity and water dispersibility of such WDPIs have been surprisingly improved. In addition, the WDPIs according to the present invention are more favorable for use in an aqueous resin as a crosslinking agent, compared with commercial product. Particularly, the aqueous resin adhesives crosslinked with the water dispersible polyisocyanate compositions of the present invention have increased adhesion and heat resistance.